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Thursday, 1 December 2011

Cavity Filter Episode 2

I was unable to gain access to a drill press so I had to use a cheap hand-drill to drill the holes in the tubes. I hated every minute of it! Our workshop has a drill press, unfortunately I didn't get access to it.

After I finished soldering the individual cavities together it was time to prepare the connectors and hairpin couplers. For that purpose I used a few pieces of 0.141" hard-line and extracted the centre conductor. The PTFE dielectric was cut into short pieces and provided spacers to keep the hairpin couplers from touching the cavity walls.

I used a hot air gun to solder the tubes together and got very nice joints using this method.

Instead of drilling holes in the end plate I notched the bottom of the tubes, bent the hairpins through the notch and soldered them before soldering the base plate in place.

The thick copper bars have almost been a show-stopper for me since it was impossible to drill holes for the hair pin couplers. Luckily, Sam recommended me to not drill holes at all and rather notch the tube and bend the hair pin through the notch. This worked very well.

The tubes were then placed on top of the bottom plate. I placed a ring of solder wire on the inside of each cavity while and simply fed solder to the outside joints as the bottom plate was hot enough to melt it. Added lots of flux and the result was excellent.

The second resonator from the right sticks too far into the cavity and will probably render it useless. I'll have to see how it affects the amplitude response.

If any of the resonators is lopsided, it can easily be bent back into the centre by sliding a drill bit into it and gently pulling it to where you want it to be.

I think I forgot to mention that I threaded the holes for the tuning screws. That way I will only need one nut to fixate them once the cavity is tuned. The screws I used are M4 x 40 but I think 25 mm ones would have done the trick too.

The annoying part began when I wanted to attach the top plate next evening.